As the functionality and bandwidth of communications networks continues to increase, the power dissipation of the network elements continues to rise concurrently. This has resulted in the use of larger forced-air thermal management systems to cool this equipment which consume more power and generate increasing levels of acoustic noise. These issues have become an area of concern for some customers.
Aerodynamic noise in a fan-cooled enclosure for electronics constitutes a difficult to eliminate noise source since it is generated by a material flow necessary for cooling the electronics. Reduction of flow rates carries the associated risk of inadequate cooling resulting in equipment damage, failure, or shortened operational life.
Noise attenuation approaches via absorption, for example with foam or other acoustic deadening insulation, are generally not possible due to space limitations within and surrounding the enclosure. Design practices which seek to eliminate particular aerodynamic noise sources such as sharp corners, edges, and cavities, are not practical or cost-effective for sheet metal enclosures intended to maximize the interior space available for electronics. Modifications to the fans via provision of serrated trailing edges on impeller blades and turbulence generators do not provide significant noise reduction unless taken to the extent that air delivery performance of the impeller is compromised.
As disclosed in U.S. Pat. No. 5,777,897 (1998) to Giorgio, brushless direct current (DC) fans lend themselves to cooling applications where it is desirable to control the fan speed. The speed and resulting airflow of DC fans is proportional to the applied voltage, therefore the fan speed may be reduced when cooling requirements lessen. Giorgio goes on to disclose a control system which matches fan speed to ambient temperature and electrical load for a system, based upon the premise that an increase in either will necessitate an increase in fan speed, whereas a drop in either would allow the fans to function at a lower, therefore quieter, level. A problem with this approach may be the need to set the fan power according to the worse-case conditions for cooling context, such as dirty air filter, highest altitude, and worse-power card in order to ensure that the fan speed is adequate for these operating conditions.
U.S. Pat. No. 5,484,012 (1996) to Hiratsuka discloses an electronic system with two fans. The electronic apparatus to be cooled is located in a housing having an intake port and an exhaust port. A cooling fan is mounted in the exhaust port, and an auxiliary cooling fan is placed near the electronic apparatus generating the heat within the housing. A first control subsystem controls the fan speed for the exhaust cooling fan in accordance with the temperature of the incoming air, and a second control subsystem controls the speed of the auxiliary cooling fan. The second control subsystem imposes a high speed on the auxiliary fan when the exhaust fan is running at a low speed, and imposes a low speed or stopped state on the auxiliary fan when the exhaust fan is running at a high speed. By reducing or eliminating the rotation of the auxiliary fan, noise is reduced. As outlined for the Giorgio disclosure, a problem with this approach may be the need to have the first control subsystem operate the cooling fan at a speed appropriate for worse-case conditions at a given ambient temperature in order to ensure that the fan speed is sufficient for those conditions.
U.S. Pat. No. 6,037,732 (2000) to Alfano et al. discloses a method for power management in a fan cooled system by controlling the operation of a brushless DC fan according to the temperature of a system. If the system temperature is below a certain value, the fan is shut down or operated at a minimum speed, and for temperatures above the certain value the fan is run at a speed proportional to the system temperature. The net result is to conserve power consumed by the fan motor. A problem with this approach may be the use of system temperature, a temperature which may not reflect the hottest or most thermally stressed component in a given operating state.
In view of the foregoing, it would be desirable to provide a technique for acoustic noise reduction by fan speed control which overcomes the above-described inadequacies and shortcomings by providing a mechanism which does adjust fan speed according to the operating conditions and needs of thermally sensitive components but does not need to overcompensate to accommodate worse-case conditions.